Electrically conductive coating



June 1956 w. P. BARROWS 2,748,701

ELECTRICALLY CONDUCTIVE comma Filed Aug. 28 1951 INVENTOR WENDELL P. BAR/POMS BY W ATTORNEYS ELECTRICALLY CONDUCTIVE COATING Wendell P. Barrows, Washington, D. C.

Application August 28, 1951, Serial No. 244,085

9 Claims. (Cl. 102-43) (Granted under Title 35, U. S. Code (1952), see. 266) This invention relates to protective coatings and more particularly to a protective coating which will conduct electric current therethrough from one side of the coating to the other.

The development of an electrically conductive coating is particularly important in the field of ordnance when it is desired to use steel as a substitute for the usual brass cartridge cases and an inexpensive but corrosion resistant coating must be provided therefor. Such coating in addition to being corrosion resistant must be sufficiently conductive as to effectively ground the case to the gun breech so that the electric primer now in common use may be grounded therethrough, and also so that hazardous static charges which might accumulate on a nonconductive coating may be dissipated. For this work such a coating must have adequate electrical conductivity, must adhere properly to the shell, must be sufficiently hard to resist marring and abrasion, should preferably have thermosetting properties which resist sticking at high temperatures, and must be compatible with the type of explosive conventionally used in such shells. It is particularly important that the conductive coating be reliable in operation so that the possibility of a delayed fire or a misfire of the projectile can be avoided.

Electrically conductive coatings of the type heretofore proposed comprise substantial amounts of powdered conductive material such, for example, as graphite or powdered metal dispersed throughout the coating whereby current passes from particle to particle to provide a continuous electrically conductive surface. The necessarily large amounts of conductive materials used and the nature of the materials added have produced coating materials which are lacking in one or more of the essential protective properties of a coating for cartridge cases. It has been found that electrically conductive coating materials presently employed are particularly deficient in corrosion protecting properties due to the fact that coatings which provide a continuous electrically conductive surface are particularly susceptible to corrosion.

The present invention provides an electrically conductive coating material which possesses the necessary protecting properties so that it may be used in coating steel cartridge cases. The coating consists of a plastic layer containing zinc particles of a selected size so that they extend entirely through the plastic layer so as to lie in contact with the case on one side and to project above the coating for engagement with the breech on the other side. By this means only a relatively small number of particles need by employed and the maximum area of lacquer may be used.

The firing current therefore passes from the primer to the metal casing, and through the zinc particles to the gun breech, while at the same time the plastic layer provides the coating with the necessary corrosion protection, adherence, abrasion resistance, and thermosetting properties. Thus the present electrically conductive coating eliminates the necessity of heavily loading the coating with conductive materials and instead, adds a rela- Fatented June 5, 1956 tively small amount of metal particles which do not alter the original protecting qualities of the plastic layer.

Accordingly an object of the present invention is the provision of an electrically conductive corrosion resistant coating.

Another object is to provide an organic vehicle containing electrically conductive particles in such small quantities that the original properties of the vehicle are not altered.

A further object of the invention is the provision of a coating containing electrically conductive particles which do not aifect the hardness, abrasion resistance, corrosion protection, compatibility with explosives, or thermosetting properties thereof.

A still further object is to provide a steel cartridge case coating containing electrically conductive particles which do not accelerate the corrosion of the steel.

Other objects and their attendant advantages will be readily apparent from the following detailed description when read in conjunction with the attached drawings wherein:

Fig. l is a perspective view of a cartridge casing utilizing a coating which is one embodiment of the present invention;

Fig. 2 is an enlarged cross sectional view of a portion of the coated casing taken on a line substantially corresponding to line 22 of Fig. l; and

Fig. 3 is an enlarged front view of a portion of the outer surface of the casing in the embodiment of Fig. 1.

Referring now to the drawings wherein like reference numerals designate like or corresponding parts throughout the several views, there is shown in Fig. 1 a cartridge case it which consists of an inner casing 12 of metal, such as steel or the like and a protective organic coating 13 distributed uniformly over the surfaces of the casing. The coating consists of an organic vehicle or base 14, such as phenolic varnish or the like, and a plurality of widely dispersed, electrically conductive beads or particles 15 having a predetermined controlled size whereby the particles 15 are somewhat larger than the thickness of the base 14. For the purpose of this invention the term widely dispersed shall be interpreted as representing a dispersion of conductive particles randomly disposed and spaced apart from each other within the vehicle of a sufiiciently small number per unit area to not materially destroy the protective properties of the coating.

Phenolic varnish or modifications thereof possesses the desirable qualities of good adhesion, abrasion resistance, corrosion protection, compatibility with explosives, and thermosetting properties which resist sticking at high temperatures, and is thus an excellent protective material for cartridge casings. However the addition of large quantities of metal powders and the like to this varnish ordinarily results in the destruction of one or more of the above noted desirable qualities. It has been found that the addition of a relatively small number of particles 15, preferably of metal such as zinc or manganese, neither accelerates corrosion nor alters the other desirable qualities of the phenolic varnish, due to the electro-chemical properties of the metals added, and to the relatively small amount of metal needed in the coating.

The particles 15 are of such size that while the inner surfaces of the particles are in contact with the casing 12 the outer surfaces thereof protrude beyond the layer in order to make electrical contact with the inner surface of a gun breech when the casing is in place therein. Electric current for controlling the ignition system flows from the casing 12 through the zinc particles 15 and then to the gun breech, but does not pass from particle to particle along the surface of the coating.

Before applying the coating 13 to the casing 12, the casings are cleaned in a hot alkaline solution and rinsed in hot water to insure that the surfaces to be coated are free from dirt and grease. If a heavy coating of rust is present, it may be necessary to pickle the casing in a solution of equal parts of water and concentrated hydrochloric acid, whereas lighter coatings of rust are eliminated by employing a solution of water and phosphoric acid. After the pickling treatment the casing is rinsed in cold water, treated in a solution of water and chromic acid, and dried by any suitable means such as an air blast. The coating 13 is applied to the casing as soon as possible after the casing has dried so that the surfaces to be coated will be free from contamination.

The coating may contain from to 40 grams of zinc particles per liter of phenolic varnish, however a solution of grams of zinc per liter is preferred, since the use of 5 grams of zinc will necessitate constant addition of zinc particles to maintain the required concentration, and the use of grams of zinc is considerably more than necessary. The coating may be applied to the casing either by spraying or by dipping with equally satisfactory results, however, since the zinc particles tend to precipitate very readily from the solution, use of either process will require constant agitation of the coating solution to maintain the particles in suspension. This may be accomplished by mechanically stirring the solution in a dip tank if the dipping process is employed or by mechanical agitation of the reservoir of a spray gun if a spraying process is used. In either case the container for the coating solution should be of a suitable corrosion resistant material such as stainless steel, and care must be taken to insure a uniform coating in which the zinc particles are distributed equally through the coating layer. After the coating 13 is applied, either by spraying of by dipping, the coated casing is baked in a conventional manner and then cooled slowly. Although the casing 12 is shown as having the electrically conductive coating applied to both its inner and outer surface, the coating on the inner surface does not affect the electrical circuit controlling the firing system. Therefore, it is obvious that a nonconducting protective coating may be applied to the inner surface of the casing and a conductive coating applied to the outer surface, which would result in a reduction in the cost of the coating material but, at the same time, simultaneous application of two different coatings would require a more complicated coating apparatus.

It is apparent that a coating of the type described will have applications other than as a protective coating for cartridge casings. For instance, such a coating may be used as a safety precaution to prevent the accumulation of static electricity on metal surfaces where corrosion protection is necessary. Various other modifications are contemplated and may be resorted to by those skilled in the art without departing from the spirit and scope of the invention as hereinafter defined by the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. An electricaly conductive coating comprising a nonconducting vehicle, and a number of conductive particles of a size greater than the thickness of said vehicle when applied to a surface widely dispersed throughout the vehicle so that electric current can flow through the particles to the surface but cannot flow from particle to particle for any substantial distance along the surface.

2. An electrically conductive coating for metal surfaces comprising a phenolic varnish, and conductive particles having a controlled size greater than the thickness of said varnish when applied to the surface and Widely dispersed throughout the varnish layer so that electric current flows through the particles to the surface.

3. A coating for metal cartridge cases comprising an organic coating layer applied to the surfaces of the case, and zinc particles having a predetermined size greater than the thickness of said layer and being dispersed wide- 1y throughout the layer so that electric current flows through the particles to the metal surface of the case.

4. An organic coating for metal cases comprising a phenolic varnish applied to the surfaces of said case, and zinc particles having a controlled size greater than the thickness of said varnish and dispersed widely throughout said varnish so that electric current flows through the zinc particles to render the coating electrically conductive transversely therethrough.

5. A coating solution for metal surfaces comprising a vehicle, and from 5 to 40 grams of zinc particles per liter of vehicle suspended in said solution, said zinc particles having a predetermined controlled size so that when the solution is applied to the metal surface the particles provide electrical contact with the metal surface and extend through and protrude beyond the varnish layer so that electric current flows through the particles to the metal surface.

6. A coating solution for metal surfaces comprising a phenolic varnish vehicle in which is suspended zinc particles in a proportion of approximately 20 grams of zinc per liter of varnish, said zinc particles having a predetermined controlled size so that when the solution is applied to a metal surface the zinc particles contact the metal surface and extend through and protrude beyond the varnish layer so as to electrically interconnect the metal surface and an external conducting surface in contact with the zinc particles.

7. A coating for a metal surface comprising an organic coating layer applied in a predetermined thickness to a metal surface, and electrically conductive particles having a predetermined size greater than the thickness of said layer and being randomly dispersed and spaced apart from each other to form minute projections above the surface of the layer and making contact with the metal surface through the layer.

8. An electrically conductive corrosion resistant coating comprising a layer of non-conducting vehicle applied to a metal surface, and a number of conductive particles having a predetermined size greater than the thickness of said nonconducting layer and being randomly distributed and spaced apart from each other to form minute projections extending from the metal surface through the layer to a point above the surface of the layer of nonconducting vehicle.

9. An organic coating for metal cases comprising a layer of phenolic varnish applied to the surfaces of the case, and conductive particles having a size greater than the thickness of the layer of varnish dispersed therein, said particles being noncontiguous with respect to each other and protruding from contact with the case through the varnish layer to form minute projections above the surface of the layer of varnish so that an electric current may flow through the conductive particles and so that the coating is electrically conductive transversely therethrough.

Refcrenccs Cited in the file of this patent UNITED STATES PATENTS 682,174 Coleman Sept. 10, 1901 2,509,875 McDonald May 30, 1950 2,568,188 Fisher Sept. 18, 1951 

8. AN ELECTRICALLY CONDUCTIVE CORROSION RESISTANT COATING COMPRISING A LAYER OF NON-CONDUCTING VEHICLE APPLIED TO A METAL SURFACE, AND A NUMBER OF CONDUCTIVE PARTICLES HAVING A PREDETERMINED SIZE GREATER THAN THE THICKNESS OF SAID NONCONDUCTING LAYER AND BEING RANDOMLY DISTRIBUTED AND SPACED APART FROM EACH OTHER TO FORM MINUTE PROJECTIONS EXTENDING FROM THE METAL SURFACE THROUGH THE LAYER TO A POINT ABOVE THE SURFACE OF THE LAYER OF NONCONDUCTING VEHICLE. 